Grain refinement of alloys using magnetic field processing

ABSTRACT

A method for refining the grain size of alloys which undergo ferromagnetic to paramagnetic phase transformation and an alloy produced therefrom. By subjecting the alloy to a timed application of a strong magnetic field, the temperature of phase boundaries can be shifted enabling phase transformations at lower temperatures.

This application claims the benefit of U.S. Provisional Application60/340,311 filed Dec. 14, 2001

FIELD OF THE INVENTION

The present invention relates to the production of refined grainstructures in structural alloys. The refined grain structures are usefulin designing superior structural alloys with step-out combination ofmechanical properties such as strength and toughness. The inventionincludes the application of a high strength magnetic field to shift thephase boundaries of alloys and thereby induce phase transformation. Themethod includes the alternate application and cessation or decrease instrength of such magnetic field and the attendant rapid forward andreverse phase transformation leading to progressive refinement of theinitial coarse grain structure of the alloy into fine equiaxial grains.Equiaxial or equiaxed grains or crystallites have approximately equaldimensions in the three coordinate directions.

BACKGROUND OF THE INVENTION

Increasing the strength of structural alloys is highly desired as itallows thinner wall construction for load bearing structural members orfor vessels used for containing pressurized fluids. Thinner wallconstruction can lead to significant economic incentives due tomaterial, fabrication, transportation and erection cost savings. Inother applications, high strength structural materials provide enablingtechnologies, for instance, structural steel components for ultra-deepwater drilling and production of hydrocarbons. However, before thestrength potential of a higher strength structural material or alloy canbe fully utilized in engineering design, it is critical that thematerial possesses adequate toughness to resist brittle fracture. It isknown to those skilled in the art that, in the case of structuralalloys, reducing the alloy's grain size can enhance simultaneously boththe strength and toughness properties.

There are a number of approaches adopted in the past to refine the grainsize of structural alloys. All of these approaches are based oncontrolled nucleation and growth of fresh grains via thermal orthermo-mechanical means to alter the stability of phases and/or bymaking the existing phases unstable.

In one commonly used approach, for example, temperature ormaterial-chemistry is changed to move the material from one phaseregion, across existing phase boundaries, into another phase region.Each of the phase regions may have one or more stable phases. In theseprocesses, however, the phase boundary and the phase free energies arenot fundamentally altered.

For instance, in one approach, refinement of the alloy grain size isachieved by inducing phase transformation via thermal cycling the alloyacross phase boundaries. Such thermal cycling treatments have been usedeffectively for grain refinement in several Fe—Mn and Fe—Ni steels usedin cryogenic applications. For instance, U.S. Pat. No. 4,257,808describes a thermal cycling treatment method for producing ultra-finegrain structure in low Mn alloy steel for cryogenic service. Thetechnical and scientific basis for thermal cycling treatment is alsodescribed in the publication, “Grain Refinement Through Thermal Cyclingin an Fe—Ni—Ti Cryogenic Alloy”, S. Jin et al., MetallurgicalTransactions A, vol. 6A, 1975, pp. 141-149. This thermal cycling methoduses existing phase boundaries. The phase boundary is not altered, noris the phase free energy changed.

U.S. Pat. No. 5,413,649 proposes cycling the temperature betweendifferent phase regions of one of the components in a compositematerial. This induces phase transformation in that component, andprovides grain refinement and superplasticity. This method uses existingphase boundary. The phase boundary is not altered, nor is the phase freeenergy changed.

In another widely used approach in high strength low alloy steels,austenite grains are refined by multi-step controlled hot workingprocess, such as hot rolling, at sufficiently high temperatures toinduce dynamic and/or static recrystallization to progressively refinethe initial coarse austenite grains. Since this involves simultaneousapplication of both heat and mechanical deformation, this approach isalso known as thermo-mechanical treatment (TMT) or processing. In mostinstances of TMT processing, microalloying with grain growth restrainingalloy additions such as Nb or mixtures of Nb, Ti are used to furthercontrol the recrystallization and subsequent growth of therecrystallized grain. Numerous patents and publications are in the artdescribing both the science and practice of this technology fordesigning commercially attractive alloys with superior structuralproperties. For example, technical publication,“Processing-Thermomechanical Controlled Processing” by I. Kozasu, pp.183-217 in “Materials Science and Technology” series edited by R. W.Cahn et al. in volume 7 “Constitution and Properties of Steels” editedby F. B. Pickering and published in 1992 by VCH, New York, provides themechanisms and processes related to TMT. U.S. Pat. No. 6,254,698“Ultra-High Strength Ausaged Steels with Excellent Cryogenic TemperatureToughness” describes the use of specific TMT to produce ultra-fineaustenite grains.

There are also other approaches for refining grain size. This includesthe cold work followed by high temperature annealing to recrystallizethe heavily deformed grains. There is no phase transformation involvedin this case; new grains of the same crystal structure nucleate and growto replace the heavily deformed, unstable grains from the cold work.Since this is a thermally activated process, higher temperaturesaccelerate the formation of new grains. For instance, U.S. Pat. No.5,534,085 proposes forging an alloy at low temperature, then heating thealloy to high temperature where recrystallization occurs to release thestored strain energy, thus achieving a fine and uniform microstructure.This process does not involve phase transformation.

U.S. Pat. No. 5,080,727 proposes heating a plastically deformed materialto high temperature that destabilizes the low temperature phase. Thisresults in a fine microstructure due to phase transformation inducedrecrystallization (presumably with increased kinetics driven by thestored strain energy). This method uses existing phase boundaries. Thephase boundary is not altered, nor is the phase free energies changed.

U.S. Pat. No. 6,042,661 proposes changing the material chemistry to moveit from an initial phase region into a different phase region, thusinducing phase transformation that results in superplasticity. Again,this method uses existing phase boundaries. The phase boundary is notaltered, nor is the phase free energies changed.

U.S. Pat. No. 3,723,194 proposes rapidly heating a material from itsinitial α state to a temperature inside the α+γ dual phase region, thusinducing instability that provides superplasticity. This method usesexisting phase boundary. The phase boundary is not altered, nor is thephase free energies changed.

U.S. Pat. No. 5,087,301 proposes rapidly cooling a molten alloy to forma solid supersaturated with a specific solute. The alloy is subsequentlyheated to a higher temperature (presumably to provide solute atoms withsufficient diffusivity) at which the solute precipitates out in the formof intermetallic particles. This process does not involve phasetransformation.

U.S. Pat. No. 4,466,842 proposes hot rolling steel when cooling from γto α+γ dual phase regions. This results in fine grain size due to twosimultaneous processes, which include the γ to α phase transformationand the strain induced γ recrystallization. This method uses an existingphase boundary. The phase boundary is not altered, nor is the phase freeenergy changed.

The limitation with current methods for grain refining is concerned withthe conflicting requirements for efficient and uniform grain refinement:high nucleation rate for new grains and no grain growth. A highnucleation rate is promoted by high thermodynamic driving force. Forthis, a large temperature change, ΔT, is required. To avoid graingrowth, the temperature change should be instantaneous. However, this isvery difficult to achieve in practice in large components that typifycommercial applications. For these components the temperature change isonly gradual even with the state-of-the-art commercial heating orcooling processes. The gradual change in temperature results innucleation of some new grains of the new phase at the early stages ofthis temperature change. Upon continued change in temperature, the alloyor material transitions more into the new phase primarily by the growthof the existing nuclei to fairly coarse sizes, which is favored overfurther nucleation. Thus, rapid heating or cooling of the material isrequired to fully take advantage of all the driving force resulting fromtemperature change to promote nucleation and discourage growth. However,due to the limitations of finite heating and cooling rates in actualpractice, the smallest grain size achievable by state-of-the-arttechniques is limited to about 10 micrometers for equiaxed grains. Thereis considerable technological interest in further refining the grainsdown to less than 10 micrometers, preferably to less than about 5micrometers, and even more preferably to less than about 1 micron. A newmaterial processing methodology without the aforementioned limitationsof current techniques is required to produce grain size refinement toless than 10 micrometers.

SUMMARY OF THE INVENTION

The invention includes a method for refining the grain size by applyinga magnetic field in alloys to reversibly induce phase transitionsbetween ferromagnetic and paramagnetic phases. Other magnetic phases areenvisioned but less preferred. This phase transformation can be inducedby changes in application of a magnetic field with or without a changein temperature. This invention is based on the effect of a magneticfield fundamentally lowering the free energies and enhancing thethermodynamic stability of the ferromagnetic phase(s), resulting inshifting of the phase boundaries. For this invention the two phases(e.g., ferromagnetic and paramagnetic phases) have different chemistriesand/or preferably different crystalline structures and transition fromone phase to the other phase requires a chemistry (e.g., precipitates)and/or crystalline structure change. The magnetic field is applied andceased or decreased for one or more cycles to obtain the desiredequiaxed grain size. The number of cycles is preferably less than 100,more preferably less than 10, even more preferably less than 5. The timebetween cycles is preferably about the same as the time the magneticfield is applied, but can be up to 10 times shorter or greater. Rampingtime during increasing or decreasing the magnetic field is preferablyminimized. Ramp up and ramp down times for 5%←→95% of the peak magneticfield are preferably less than 10 seconds, more preferably less than 5seconds, and even more preferably less than 1 second. The magnetic fieldcan be stepped up and/or down (preferably in one step) or ramped upand/or down. For example as seen in FIG. 4, the magnetic field can beincreased and/or decreased in either a single or multiple steps. Thephase boundary temperature is shifted up (with increasing magneticfield) or reverted (with decreasing magnetic field) so that theequilibrium ratio of different phases changes. Ratios can be measured byvolume ratios, wherein a single phase has a ration of, for example,100%:0%. Hence, the invention is directed to a method for refining theequiaxed grain size of an alloy which undergoes a ferromagnetic toparamagnetic transition comprising (a) subjecting said alloy to amagnetic field of a sufficient strength and for a time sufficient tocause said alloy to transition from its original initial phase ratio(condition A) to a new phase ratio (condition B), and (b) decreasingsaid magnetic field to allow said alloy to transition to yet a differentphase ratio (condition C), wherein said condition C may be the same ordifferent from said condition A, and optionally repeating steps (a) and(b). The decreasing of magnetic field in (b) may include reducing themagnetic field to zero as well as changing it to a strength differentfrom that in (a).

The invention produces a metal or alloy, at the high temperature chosenfor magnetic processing, having fine equiaxed grain size of less than 10micrometers, preferably less than about 5 micrometers, and even morepreferably less than about 1 micron. In a preferred embodiment, thealloy is cooled (e.g., ambient air cooling, fast quenching in a fluidmedium, accelerated cooling in a medium) after magnetic processing tobelow about 500-550° C. to minimize grain growth. In another embodiment,said fine equiaxed grain metal or alloy can be subjected to subsequentprocessing by conventional methods to further reduce the grain size.Said conventional processing includes high temperature processing (e.g.,thermo-mechanical controlled processing—TMCP, hot rolling, hot bending,hot forging, etc.) and cooling from high temperature to ambient or sometemperature in between. In addition to grain size and shape, thematerials produced by this invention can have improved graindistribution, and surfaces.

Furthermore, the invention is broadly directed to metals or alloys whichundergo ferromagnetic to paramagnetic phase transitions. The inventionis preferably suited to alloys of Fe, Ni, and Co, individually or incombination (e.g., Fe—Ni—Co alloys), and with or without carbon.Impurities or minor alloying may be allowed per conventional engineeringpractice. Without limiting this invention, said impurities or minoralloying may include S, P, Si, O, N, Al, etc. In particular, thisinvention is suited for carbon and low alloy steels including highstrength low alloy (HSLA) steels. For the purpose of this invention,HSLA steels are Fe based steels with less than about 8 wt % totalalloying content.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the Fe—C phase diagram and a schematic depicting the priorart approaches to refine the grain structure of austenite or gamma (γ)phase at high temperature.

FIGS. 2 and 3 depict the present invention using Fe—C alloy (carbonsteel) as an example.

FIG. 4 shows example experimental results according to the presentinvention with an AISI 1018 carbon steel at a constant 764° C.temperature were the application and removal of magnetic field isplotted against duration of the exposure of the steel to the magneticfield. The magnetic field is ramped in steps to a maximum of 19 tesla(T). The circular data points are the experimentally measured linear %expansion data points using the dimension of the steel bar at 764° C.without the magnetic field as a reference point.

FIG. 5 shows the Fe—C phase diagram and examples of the preferred alloycomposition range for practicing the current invention to maximize thegrain refining effect.

DETAILED DESCRIPTION OF THE INVENTION

Although the embodiments of the present invention are described in thefollowing using its application to carbon and low alloy steels, it wouldbe obvious to those skilled in the art that the invention has broadapplicability to any alloy which displays magnetic phase transitions,preferably ferromagnetic ←∵ paramagnetic phase transitions. The alloysof the invention with refined equiaxed grain size which are produced bythe invention described herein may be used to fabricate structuralcomponents and processing equipment such as pressure vessels. Thesestructures and equipment have applications such as in oil and gasexploration, oil and gas production, refining processing, and chemicalprocessing. The refined grain alloys produced herein provide strongerand tougher materials out of which structural components can befabricated. Beneficially, alloys with equiaxed grain size of less than10 micrometers at high temperature can be produced. Said alloys can befurther processed by conventional methods including high temperatureprocessing (e.g., TMCP, and other hot deformation such as rolling,bending, forging, etc.) and cooling to ambient or other temperature inbetween.

In prior art approaches, repeated thermal cycling to change a phaseratio, for instance, between the single phase γ and two phase ferrite(α)+γ regions across existing phase boundaries of a carbon steel willlead to production of a certain α to γ phase ratio and its revertingback to form 100% γ phase in one thermal cycle. This forward and reversephase transformations take place by nucleation and growth of the stablephase consuming the unstable phase. These repetitions produce the grainrefining depicted in the schematic of FIG. 1. Each time there is anucleation stage, typically there are more than one nuclei formedthereby breaking up the pre-existing grains into smaller units orgrains. Upon repeated thermal cycling across the phase boundary regions,the original coarse grain structure is broken up into fine grains asshown in the schematic of FIG. 1. The state-of-the-art technology islimited to equiaxed grain size refinement to about 10 micrometers (atthe processing temperature) due to the limitations in rapidity withwhich the thermal cycles can be accomplished in existing commercial heattreatment facilities. This is primarily limited by the time required forheat-up and cool-down cycles and the ensuing growth of existing grainsover fresh nucleation during this time period.

In the present invention, the phase transitions between two differentphase regions are accomplished at a temperature preferably no more thanabout 100° C. above the curie temperature (T_(C)). In the absence of anexternal magnetic field a ferromagnetic material becomes paramagneticabove the Curie temperature. In the α+γ phase region of steel shown inphase diagrams, it is also possible to move within the same phase regionbut with differing volume fractions or phase ratios of the constituentphases. The temperature may be fixed or may vary within the noted rangeduring application of the magnetic field. Therefore, the temperatureduring application of the magnetic field can be fixed at any temperaturefrom A₁ up to a temperature equal to T_(C) plus 100° C. or may varywithin this range. A₁ for steels is the temperature of the boundarybetween the α+γ phase region and the α or α+Fe₃C phase region. A₃ forsteels is the temperature of the boundary between the α+γ phase regionand the γ phase region. More preferably, the maximum temperature forapplication of the magnetic field will be no greater than T_(C) plus 50°C. The strength of the magnetic field to be applied to the alloy will begreater than 2 T (depending on the alloy), preferably greater than 5 T,more preferably greater than 10 T, even more preferably greater than 20T, and most preferably greater than 50 T. The magnetic field is believedto cause the alloy's phase boundary to shift by affecting the Gibb'sfree energies of the ferromagnetic phases. As a result of the phaseboundary shift, new crystallization nuclei of the stabilized phase areformed thereby breaking existing grains into smaller equiaxed grainscausing grain size refinement. This invention is based on magnetic fieldinduced nucleation and growth of new grains. This is preferably inducedby For steels, α is a phase that has a body centered cubic (BCC)crystalline structure (or some distortion of BCC) and is ferromagneticbelow its Curie temperature, but becomes paramagnetic above its Curietemperature. A typical Curie temperature for carbon steels is about 770°C. Also for steels, γ is another phase that has a face centered cubic(FCC) crystalline structure and is paramagnetic. These two phases havedifferent densities.

The invention is more easily understood by reference to the schematicFe—C steel phase diagram shown in FIGS. 2 and 3. In the presentinvention, the alloy to be subjected to a magnetic field can initiallybe in any phase boundary region provided the initial phase boundaryregion is within A₁ to T_(C)+100° C. In this invention magneticfield-induced phase boundary shifting accomplish the advantageous phasetransformations to maximize breaking up of initial coarse grainstructures into fine crystallites/grains. One embodiment of the presentinvention involves applying or changing a magnetic field at a fixedtemperature. In another embodiment of the present invention thetemperature can be changed while applying a fixed or varying magneticfield. For example, a magnetic field can be applied while a steel alloyis cooling.

FIGS. 2 and 3 exemplify an application of the present invention. Thephase boundary shift taught herein can be accomplished in thetemperature range between the solid horizontal A₁ line and T_(C)+100° C.(T_(C) is the Curie temperature). More preferably, this can beaccomplished in the two temperature regions that are respectively abovethe A₁ as shown in FIG. 2, and close to the solid A₃ sloped line asshown in FIG. 3. At the lower temperature region near A₁, in the absenceof a magnetic field, the steel undergoes a transition from α+γ two phaseregion to α+Fe₃C phases upon cooling from a temperature above A₁ throughA₁. In the higher temperature region near A₃, in the absence of amagnetic field, the steel undergoes phase transition from the singlephase γ to two phases α+γ upon cooling from a temperature above A₃through the A₃ temperature. The corresponding reverse phasetransformations occur during heating through A₁ and A₃ temperatures,respectively. While cooling is the economically preferred process,similar heating schemes can also induce phase transition, though in thereverse direction. In FIGS. 2 and 3, the dashed lines depictschematically the shifted location of the A₁ and A₃ temperatures withthe application of a magnetic field in accordance with the presentinvention. In FIG. 2(a) the solid circle at 0.4 wt % carbon andapproximately 740° C., represents the initial steel condition beforeapplication of any magnetic field. Upon application of the magneticfield, the A₁ phase boundary is shifted upwards from the horizontalsolid line to the horizontal dashed line. As a result of turning on themagnetic field, the steel held at constant temperature now is in theα+Fe₃C region instead of the α+γ region. By turning off the magneticfield, the steel is reverted back to the α+γ region. This process can berepeated multiple times as necessary. FIG. 2(b) depicts schematicallythe refinement of initial grain size upon repeated application andcessation of magnetic field to an Fe—C steel initially (as shown by thesolid circle) at a temperature near the A₁ temperature. In FIG. 3(a) thesolid circle at 0.4 wt % carbon and approximately 830° C., representsthe initial steel condition before application of any magnetic field.Upon application of the magnetic field, the A₃ phase boundary is shiftedupwards from the sloped solid line to the curved dashed line. As aresult of turning on the magnetic field, the steel held at constanttemperature now is in the α+γ region instead of the γ region. By turningoff the magnetic field, the steel is reverted back to the γ region. Thisprocess can be repeated multiple times as necessary. The schematic inFIG. 3(b) depicts the refinement of initial grain size upon repeatedapplication and cessation of the magnetic field to an Fe—C steelinitially (as shown by the solid circle) at a temperature near the A₃temperature.

Applicants believe that the shifting between two different phase ratioswith the application of magnetic field allows for grain size refinement.Hence, for example, the alloy to be acted upon can be in the 100% γphase and as a result of application of the magnetic field can shiftinto a certain α:γ phase ratio and then back upon ceasing or reducingstrength of the magnetic field applied; for example see FIG. 3. Thealloy could likewise start out in the α+γ phase and be shifted to thepredominantly α phase (with some Fe₃C) as a result of magnetic field andthen back; for example see FIG. 2. All that is necessary is that thealloy be cycled between two points in the phase diagram that havedifferent ratios (e.g., volume fractions) of α and γ phases. The shiftneed not be between adjacent phase boundaries; it can also beaccomplished by either or both of the following two techniques. First,by using the suitable alloy chemistry (e.g., adding alloying such ascarbon), the temperature gap between A₁ and A₃ can be narrowed. Forexample, as seen in FIG. 2, using 0.7 wt % carbon creates a gap of only20° C. Second, potentially with a very high magnetic field, it may bepossible to shift across two phase boundaries. For example, as seen inFIG. 3, the predominant steel phase could be shifted from γ to α+Fe₃Cand then back to γ or α+γ. However, a steel alloy must initially be inthe α+γ or γ phase region prior to application of the magnetic field.Preferably, the alloy will be in the γ phase region prior to applicationof the magnetic field, to take advantage of the faster phasetransformation kinetics at higher temperature.

When an α phase forms at the expense of γ phase in steels, the steelundergoes a dimensional change, in this example, an expansion due to thelower atom packing density of the body centered cubic (BCC) structure ofthe α phase compared to the higher atom packing of the face centeredcubic (FCC) crystal structure of the γ phase. Thus, the dimensionalchange can be monitored to gain an understanding of the phase(s) thatare growing at the expense of other phase(s). FIG. 4 presentsexperimental data of measured dimensional change for AISI 1018 carbonsteel, having a carbon content of about 0.18 wt %, when a magnetic fieldis applied in stages to ramp up to a maximum field strength of 19 T at aconstant temperature of 764° C. At this temperature when the steel isequilibrated, the steel is in a two-phase α+γ phase region in theabsence of a magnetic field. It can be seen that when the magnetic fieldis turned on, the steel specimen undergoes expansion, indicating thegrowth of α phase at the expense of γ phase. The amount of α phasecontinues to increase up to the maximum magnetic field studied. It canbe seen that ceasing the magnetic field can reverse the phase changes.The experiment provides confirmation that the phase stability can beinfluenced at a constant temperature by the application or cessation ofa magnetic field. In the presence of a magnetic field, the thermodynamicstability of the ferromagnetic phase, α, is increased leading to itsnucleation and growth at the expense of the paramagnetic γ phase. Theapplication and cessation of the magnetic field can be repeated a numberof times to obtain progressive grain refinement each time the field isapplied and then ceased or cycled.

In order to provide maximum grain refining efficiency, it is preferablethat at least 15 vol %, more preferably 30 vol %, even more preferably50 vol % of the steel has gone through transformation with each cycle ofthe application of the magnetic field. To maximize grain refining,magnetic cycling (either on-off or changing field strength) can beapplied.

A particular aspect of this invention is to couple suitable alloychemistry design with the application of specific magnetic fieldstrengths. This is illustrated in FIG. 5, which is a Fe—C phase diagram.As an example, if we use a steel chemistry having 0.4 wt % carbon (C),when the temperature is about A₁ (˜730° C.), a shift of 20° C. achievedwith the application of a magnetic field results in a change of greaterthan 50% change in the volume distribution of the phases. In thisexample, the steel is initially in the two-phase α+γ phase region ataround 750° C. in the absence of a magnetic field. When the magneticfield of sufficient strength is applied to cause a 20° C. upward shiftin phase boundary, about 55% by volume of the γ phase are replaced withα phase (possibly with some Fe₃C). On the other hand, if we use a steelchemistry having a lower carbon content, such as with 0.2 wt % C, thesame magnetic field induced 20° C. boundary shift results in only 28 vol% of the γ phase replaced with α phase. Thus, the grain refiningefficiency will be far more effective in the 0.4 wt % C steel than inthe 0.2 wt % C steel. The amount of phase changes for a given magneticfield strength is a function of the alloy chemistry as it relates tomagnetization. Within the general steel chemistry considerations knownin the art, it is preferable in the present invention that an alloychemistry be selected to maximize the amount of phase changes for agiven shift in the phase boundary with the magnetic field application orcessation.

The minimum time for application of a magnetic field cycle is dependenton how long it takes for sufficient metal to transform into a differentphase. The maximum time is limited by economics and the minimization ofundesired grain growth. Ideally, the magnetic field is applied for atime sufficient to complete all the desired phase transformation perthermodynamic equilibrium, but short enough before the newly formedgrains begin to grow. In practice, there is a compromise between thesetwo requirements of transformation completion and grain growth.

For example, in a manganese steel having a chemistry of 0.43C-1.6 Mn, atA₃ (roughly 750° C.) has 100 vol % γ phase (Condition A). A 50 Tmagnetic field is estimated to impart approximately a 50° C. upwardsshift in the A₃ phase boundary resulting in a phase ratio of 25 vol % γto 75 vol % α (Condition B) at thermodynamic equilibrium. It takes along time to reach thermodynamic equilibrium. It takes roughly 5 secondsto complete about 5% of the transition from Condition A to Condition B.It takes roughly 40 seconds to complete about 50% of this transitionfrom Condition A to Condition B. At this stage up to about 40 secondsthe process is dominated by nucleation. It takes roughly 2000 seconds tocomplete about 80% of the transition from Condition A to Condition B.This later stage is dominated by growth of newly formed grains.Preferred times for application of this 50 T magnetic field (i.e., tocomplete about a 50% transition from Condition A to Condition B) are atleast about 40 seconds (sec) and less than about 150 seconds (to avoidexcessive growth).

Preferred times will depend on the alloy chemistry, alloy temperature,and amount of phase boundary shift (related to magnetic field strength).Generally, it is preferred to apply the magnetic field for a sufficienttime period to maximize transformation while minimizing excessive graingrowth. While dependent on the above variables, preferred applicationtimes for applying a magnetic field are about 0.1 to about 3000 seconds,more preferably for about 0.1 to about 1000 seconds, even morepreferably about 1 to about 100 seconds. In one embodiment, this fieldis cycled with the off time about equal to the on time. In anotherembodiment, the off time is different from the on time. The examplesherein are for illustrative purposes and are not meant to be exclusiveor limiting.

Typical alloys which can be refined in accordance with the presentinvention include, but are not limited to, alloys of iron, nickel,cobalt, individually or in combination. In one of the preferredembodiments, the alloys will contain at least 92 wt % of iron, nickel,cobalt, or a combination thereof. This in these alloys, no more than 8wt % of other components are present. Most preferably, iron alloys willbe utilized as they represent technologically some of the most importantalloy systems. Some examples of preferred materials include, but are notlimited to, high strength low alloy steels such as API X80, ASTM A516grade 60 or 70 and AISI grades 1010, 1018, 1020, 1040, 4120, 4130, or4140. However, as should be obvious for those skilled in the art, thepresent invention is not limited to ferromagnetic steels, alloy steels,high strength low alloy steels, nickel alloys, and cobalt alloys. Theinvention is broadly applicable to alloys which undergo a magnetictransition such as ferromagnetic to paramagnetic transition.

The temperature of the phase boundaries as well the Curie temperaturecan be modified by alloy chemistry. Alloy chemistries are preferablydesigned to maximize phase ratio change with minimum phase boundaryshift as shown above. For example, adding nickel or cobalt to steel canchange its Curie temperature, whereas adding carbon does not. Forexample, adding nickel, carbon and/or nitrogen can depress A₃temperature. With this disclosure one can construct a phase diagram thatshows the phase region boundaries for any given alloy to design amagnetic procedure according to the invention. For instance, this may beaccomplished using THERMO-CALC software (Thermo-Calc AB, Stockholm,Sweden).

The magnetic field to be applied will be of sufficient strength to causea shift in phase boundary preferably at least by about 10° C., morepreferably at least by about 20° C., and even more preferably at leastby about 50° C. In steel, a one T magnetic field roughly causes a onedegree Celsius shift of the A₁ and A₃ phase boundaries. The magneticfield may be applied for a sufficient time to complete a percentage ofthe expected phase transformation. It is preferable to achievetransformation of at least about 15 vol %, more preferably at leastabout 30 vol %, and even more preferably at least about 50 vol % of thealloy. The maximum time the field will be applied is a time which isshorter than the time required to induce grain growth for that alloy.Hence, the strength of the magnetic field will be at least about 2 T(for certain alloys), preferably at least 10 T, more preferably at leastabout 20 T, even more preferably at least about 50 T. Increasing thenumber of magnetic field cycles (when each cycle is applied forsufficient times to achieve a percentage of the expected phasetransformation), generally leads to more refinement. Although themagnetic field is preferably ceased for as long as it takes for thealloy to return substantially to its initial phase ratio (anddimensions), shorter or longer cessation times are possible. Therefinement of the alloy during the process of this invention can bemonitored by dimensional change similar to that depicted in FIG. 4.Hence, the one can determine how long the field should be applied andceased during each cycle or repeat of steps (a) and (b). If the magneticfield is simply decreased in strength, the amount of time before themagnetic field strength is increased again will preferably be thatamount of time required for the alloy to reach phase (and dimensional)equilibrium. In practice, however, this time may be shorter, but themaximum benefit will be recognized when at least about least 15 vol %,more preferably at least about 30 vol %, even more preferably at leastabout 50 vol % of the of the alloy has undergone phase transformation.

1. A method for refining the grain size of an alloy which undergoes amagnetic field induced phase transformation, comprising: (a) subjectingthe alloy to a magnetic field of a sufficient strength and for a timesufficient to cause the alloy to transition from a first phase ratio toa second phase ratio; and (b) decreasing the magnetic field to allow thealloy to transition from the second phase ratio to a third phase ratio,wherein the third phase ratio may be the same or different from thefirst phase ratio; and optionally repeating steps (a) and (b).
 2. Amethod according to claim 1, wherein the alloy is selected from thegroup consisting of steel, iron alloys, cobalt alloys, and nickelalloys; the decrease of the magnetic field in (b) reduces the magneticfield to about zero T; and the third phase ratio is the same as thefirst phase ratio.
 3. A method according to claim 2, wherein the alloycontain at least 92 wt % of iron, cobalt, nickel, or a combinationthereof.
 4. A method according to claim 1, wherein the first phase ratioand the second phase ratio are in adjacent phase boundary regions.
 5. Amethod according to claim 1, wherein the application of the magneticfield is increased and decreased as single step changes.
 6. A methodaccording to claim 1, wherein the magnetic field has a strength greaterthan about 5 T.
 7. A method according to claim 1, wherein the methodproduces equiaxial grains having a mean grain size of less than about 10micrometers at the end of the method.
 8. A method according to claim 1,wherein the alloy changes temperature by no greater than about +/−50° C.during the method.
 9. A method according to claim 1, wherein the methodis performed at an approximately fixed temperature.
 10. A methodaccording to claim 3, wherein the first phase ratio is at a temperaturewithin the range of of about A₁ to about T_(C)+100° C.
 11. A methodaccording to claim 1, further comprising a cooling step (c) to cool thealloy to below about 500° C.
 12. A method according to claim 1, furthercomprising a hot working step (c).
 13. A high strength low alloy steel,comprising: at least about 92 wt % Fe and having a mean equiaxial grainsize of less than about 5 micrometers after application of a magneticfield of at least 5 T but without deformation or cooling.
 14. A steelaccording to claim 13, wherein the mean equiaxial grain size is lessthan about 1 micron.
 15. A method according to claim 1, wherein thealloy is a high strength low alloy steel comprising at least about 92 wt% Fe.
 16. A method for refining the grain size of an alloy, comprising:a ferromagnetic phase and a paramagnetic phase separated by a phaseboundary, comprising: (a) subjecting the alloy with a first volume ratioof the ferromagnetic phase and the paramagnetic phase, to a magneticfield of sufficient strength to cause the temperature of the phaseboundary to shift upwards, and a sufficient time to change the firstvolume ratio to a second volume ratio such that the magnetic fieldcauses at least about 15 vol % of the alloy to transform from theparamagnetic phase to the ferromagnetic phase; (b) decreasing themagnetic field to allow the alloy to transition to a third volume ratiowherein the third volume ratio may be the same or different from thefirst volume ratio; and optionally repeating steps (a) and (b).
 17. Amethod for refining the grain size of an alloy, comprising: aferromagnetic phase and a paramagnetic phase separated by a mixed phaseregion having a lower phase boundary and an upper phase boundary,comprising: (a) subjecting the alloy with a first volume ratio of theferromagnetic phase and the paramagnetic phase, to a magnetic field ofsufficient strength to cause the temperature of the phase boundary toshift upwards, and a sufficient time to change the first volume ratio toa second volume ratio such that the magnetic field causes at least about15 vol % of the alloy to transform from the paramagnetic phase to theferromagnetic phase; (b) decreasing the magnetic field to allow thealloy to transition to a third volume ratio wherein the third volumeratio may be the same or different from the first volume ratio; andoptionally repeating steps (a) and (b).
 18. A method according to claim17, wherein the third volume ratio is the same as the first volumeratio.
 19. A method according to claim 17, wherein the alloy is a iron,nickel, or cobalt alloy.
 20. A method according to claim 19, wherein thealloy is a low alloy steel with a total amount of alloying less thanabout 8 wt %.
 21. A method according to claim 20, wherein the steel is amember selected from the group consisting of API X80, ASTM A516 grade60, ASTM A516 grade 70, AISI grade 1010, AISI grade 1018, AISI grade1020, AISI grade 1040, AISI grade 4120, AISI grade 4130, and AISI grade4140.
 22. A method according to claim 21, wherein the alloy is a steel;in step (a), the magnetic field is at least about 10 T and is appliedfor a time of about 0.1 seconds to about 1000 seconds; in step (b) themagnetic field is decreased to about zero T for a time of about 0.1seconds to about 1000 seconds; and the temperature is between about A₁and about T_(C)+100° C.
 23. A method according to claim 22, wherein instep (a), the magnetic field is at least about 20 T and is applied for atime of about 1 second to about 100 seconds.
 24. A method according toclaim 23, wherein the magnetic field is cycled from 2 to about 10 timeswherein the time between magnetic cycles is about 0.1 seconds to about1000 seconds independently of the time in step (a).
 25. An alloy,wherein the grain size is refined by the method according to claim 1.